RdsSurveyor
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ChristopheJacquet
Unusual values observed for type 8 and 9 quantifiers in TMC messages
These days I observed a TMC message with a temporary width limit, which RDS Surveyor decoded as 3.4 m. As this seemed suspicious to me (maximum permissible width for road vehicles being 2.55 m, or up to 3 m for special vehicles), I dug a bit deeper and found an older recording of several messages with width and weight restrictions. Almost all of them were beyond the expected range, with width restrictions of up to 11 m and one 60 t weight restriction—the only valid-looking quantifier is for a 3.5 t weight restriction.
Looking at the data, the raw quantifier seems to be decoded correctly. 3.4 m is decoded from 34, 11 m from 102 and 60 t from 200—all of which look correct to me.
I’m wondering whether there is some bit misalignment causing this error. In most of the cases I observed this would have gone unnoticed, as the width limit is almost always the last event in the message. I have seen only two messages where the width limit is followed by another event; in one of those two, the following event is the weight limit (with an equally suspicious-looking quantifier).
I’ve taken an event from the above sample, cut out a single occurrence of that event from the RDS log and analyzed it manually. It doesn’t look like there’s any misalignment in the free-form data: the first group has the event and location in its fixed-length fields. The free-form bits in the second group start with a separator label (which has no data), an additional event label, the second event, a type 5 quantifier label, 8 bits of data (the last bit being 1) and zero bits as padding.
The 8 quantifier bits decode to 0x3D, or 61, which translates to 6.1 m, as displayed by RDS Surveyor. For all intents and purposes, the broadcaster seems to be transmitting bogus width limits. It wouldn’t be the first time I’m seeing invalid data from a broadcaster (I have in the past observed events with a zero event code and asked the broadcaster about it, and it turned out to be an encoding error).
D_1_10867_width.rds.zip TMC Width.pdf
Got feedback from the broadcaster: apparently, the width restriction applies to the entire carriageway, not to individual lanes. Considering this, the figures seem to make more sense—although they would be of little practical use. I am still waiting for an answer on the 60 t limit—in Germany, anything above 40 t (44 in certain cases) requires a special permit anyway.